What CNC Machines Are in a Fab Lab, and How Are They Specified?

The short answer

A fab lab built to the standard inventory usually has two CNC machines that have almost nothing in common: a large-format router sized for full sheets of plywood, used for furniture-scale parts and molds, and a small high-precision mill used for circuit boards and tiny features. The expected machine classes come from the Fab Foundation's planning material (Getting Started with Fab Labs) and the MIT program that originated the concept (Center for Bits and Atoms), which is why a file cut in one lab can usually be cut in another.

The big router

This is the machine people mean by fab lab CNC. It is a gantry router over a flat bed, moving a spinning cutter through sheet material.

Specs that change what you can make:

Work envelope. Many labs size the bed to take a full 4 by 8 ft sheet (roughly 1.2 by 2.4 m) because that is how plywood and MDF are sold. A bed smaller than your stock means cutting sheets down first, every time.
Z clearance and plunge depth. Clearance sets the thickest stock you can place; usable cut depth is limited by cutter length, not just the gantry.
Spindle power and rpm range. Power determines how aggressively you can cut hardwood or plastic; the rpm range has to suit the cutter diameter you actually own.
Collet size. This quietly decides which tooling fits, and tooling is the recurring cost.
Workholding. A vacuum table is fast but needs flat stock; screws into a sacrificial bed are slower and far more forgiving. Most labs run a sacrificial layer either way.
Frame rigidity. Two machines with identical envelopes can produce very different edge quality, because deflection shows up as a tapered or chattered cut.
Dust and chip extraction. On a router this is a design requirement, not an accessory, and institutions write it into their equipment policies.

The small precision mill

The desktop mill solves the opposite problem: very small features, very fine tooling, very little material removal. Its classic jobs are milling copper-clad board to make circuits and cutting wax for small molds.

What matters here:

Resolution, repeatability, and backlash, not envelope. The part is tiny; the question is whether the machine returns to the same coordinate.
Spindle runout, because a cutter measured in hundredths of an inch will snap if it is wobbling.
Bed flatness and probing, since trace milling depends on a consistent surface height across the whole board.
Tooling inventory. Engraving bits and very small end mills are consumables with a short life, and they are the main reason the machine sits idle.

Spec-sheet vocabulary worth knowing

Travel is how far each axis moves; work envelope is the usable volume once workholding and the tool are in place. They are never the same number.
Accuracy is how close the machine gets to the commanded point; repeatability is whether it misses the same way twice. Repeatability is often more useful.
Stepover and depth of cut describe how much material each pass removes, and they trade speed against finish.
Kerf on a router is simply the cutter diameter plus deflection, which is why press-fit joints need a test cut rather than trust in the CAD file.
Controller and post-processor decide whether your toolpath software can actually drive the machine. A great machine with no supported post is a doorstop.
Tool change is manual on nearly all community-scale machines; automatic changers belong to production shops.

What is usually not in a fab lab

Metal lathes, plasma tables, and waterjets are not part of the standard kit, though individual labs add them. If you came looking for metalwork, check the machine list before you drive out. The broader inventory is laid out in what equipment a standard fab lab has, and the other big-ticket machine is covered in what specs matter on a fab lab laser cutter.

Access, footprint, and training

A sheet-goods router needs clear space on all four sides for infeed and outfeed, plus room for dust collection, so it drives the floor plan more than any other machine; see how much floor space a fab lab needs. Access is almost always machine-specific: a separate sign-off per machine rather than one blanket orientation. Institutions publish their own written programs for this, and a representative example is Binghamton University's makerspace safety program. How labs sequence those sign-offs is described in makerspace equipment training and access.